Today, there is increasing hope for the practical application of fuel cell automobiles. Development of materials is being actively carried out in order to provide materials for use not only in fuel cell automobiles but in high pressure storage vessels, piping, valves, and the like to be used in hydrogen gas stations and the like which are necessary to establish an environment of use for fuel cell automobiles. Such a high pressure environment is said to be 50 MPa or above.
An austenitic stainless steel which has excellent resistance to hydrogen embrittlement is considered suitable for use in a usual hydrogen environment. Accordingly, various attempts are being made at increasing its strength to enable such an austenitic stainless steel to withstand a higher pressure hydrogen environment of 50 MPa or above. For example, it has been proposed to obtain a high strength by increasing the Mn content of the base metal, increasing the solubility of N, adding a relatively large amount of N and V, and carrying out suitable heat treatment.
However, even if it is possible to obtain a high strength base metal, it is difficult to obtain a high strength even with an austenitic weld metal. Since the weld metal undergoes melting and solidification and the weld heat affected zone undergoes heating and cooling at the time of welding, a decrease in strength occurs in welds. Therefore, in the past, strengthening methods were employed to precipitate fine grains by heat treatment after welding.
For example, Japanese Published Unexamined Patent Applications Hei 5-192785 and Hei 10-146692 disclose forming a weld metal from an Ni-base alloy to which Ti and Al have been added and heating it in a prescribed temperature range to precipitate fine intermetallic compounds (Ni3Al, Ni3Ti) referred to as gamma′ prime (γ′) phase, whereby the weld metal can be strengthened. However, these weld metals have the problem that they have a high susceptibility to weld hot cracking and that it is easy for a decrease in toughness to occur due to hardening caused by precipitation of the above-described intermetallic compounds.
Japanese Published Unexamined Patent Application Hei 9-271982 discloses that improving the composition of the coating in a coated electrode for arc welding is effective at preventing weld hot cracking in high-Ni weld metal containing Ti and Nb, but such a method is intended for buildup welding.